Mitochondrial dysfunction is often an underlying cause of myocardial disease. In particular, many cardiac pathologies are associated with rapid and dramatic increases in mitochondrial permeability. These changes in permeability instigate a molecular chain of events that leads cardiomyocyte death. Our long-range goal is to understand how specific mechanisms of mitochondrial-driven death can be targeted for the prevention of myocardial disease. The mitochondrial permeability transition (MPT) pore, a large, non-specific channel thought to span both mitochondrial membranes, is known to mediate the lethal permeability changes that initiate mitochondrial-driven death. The MPT pore was originally proposed to consist of the voltage-dependent anion channel (VDAC) in the outer membrane, the adenine nucleotide translocase (ANT) in the inner membrane, plus a regulatory protein cyclophilin-D (CypD) in the matrix. However, while we, and others, have shown that mice lacking CypD are indeed resistant to MPT and MPT-mediated cell death, mice lacking either VDAC or ANT still exhibit a classical MPT phenomenon and respond normally to cytotoxic stimuli. Consequently, with the exception of CypD, the precise molecular component of the MPT pore has still not been defined. We have identified the mitochondrial phosphate carrier (PiC) as a novel CypD-interacting protein, and have generated strong preliminary data that the PiC is a positive regulator of MPT and cell death. Consequently, our central hypothesis is that PiC is an essential component of the MPT pore, and, therefore, a critical mediator of cardiomyocyte death. The objective of the present application, therefore, is to utilize genetic gain- and loss-of-function approaches to systematically evaluate the role of the PiC in MPT, cardiac cell death, and the progression of myocardial disease. Our specific aims are as follows: Specific Aim 1: Define the physical and functional interaction between PiC and CypD; Specific Aim 2: Examine whether PiC upregulation induces MPT and cardiac pathology; and Specific Aim 3: Determine the functional requirement for PiC in MPT and cardiac cell death. The rationale for the proposed research is that once key mitochondrial proteins that participate in mitochondrial dysfunction are identified, they can be targeted as a means of treating a whole array of human cardiac diseases.